Conventional variable gain amplifiers often rely on a differential amplifier, load resistors, and cascode switching transistors to produce variable gain output either in discrete steps (digital) or continuously variable (analog) as a function of a control signal. However, such a design results in different common mode (average) voltage level associated with the variable gain outputs. This may cause performance problems with other devices connected to the differential amplifier, e.g., downstream stages.
Many conventional variable gain amplifiers may rely on AC coupling (capacitors) to connect stages connected to the variable gain amplifier to allow the DC common mode voltages at different stages to be independent. However, series capacitors may limit the low frequency response of the amplifier. Additionally, capacitors can be quite large in size.
For example, U.S. Pat. No. 4,462,003, incorporated by reference herein, discloses a variable gain amplifier which employs a switched cascode design that utilizes different load resistor configurations to provide variable gain levels. U.S. Pat. No. 6,703,899, incorporated by reference herein, teaches a quad switched gain circuit that also utilizes a switched cascode approach to achieve different levels of gain. U.S. Pat. No. 5,900,781, incorporated by reference herein, discloses a multistage variable gain amplifier circuit that employs AC coupling capacitors between stages.
The Gilbert cell is a common cell used to vary the gain of an amplifier stage in analog or digital fashion while keeping a constant differential common-mode output voltage. The Gilbert cell includes a conventional differential amplifier common-emitter (for bipolar transistors) or common-source (for field effect transistors (FETs)) transistor pair with a cascode transistor pair attached to each output of the differential amplifier. The Gilbert cell relies on the cascode pairs to deliver or bleed current to the load resistors to create variable gain. Each leg of the Gilbert cell has two active cascode devices. Such a design results in an undesirably high level of output noise as the gain of the cell is varied.